The present invention relates to a duo-servo brake. More particularly the invention relates to a mechanically actuated duo-servo parking brake employable independently or in combination with a normal disk brake in an automotive vehicle.
As in drum brakes in general, in a duo-servo brake, usually two shoes, are provided in a rotating drum mounted on a wheel unit, are fitted externally with friction material and may be forced outwards to be brought into pressure contact with the interior of the drum and thereby stop, or prevent, rotation of the drum and wheel unit.
The tips of the shoes are connected by pull-off springs to an anchor pin which is mounted in an upper central portion of the interior space defined by the drum, and may be moved away from the anchor pin, by actuation means described below, counter to the force of the pull-off springs, whereby the shoes are brought into contact with the drum. A distinguishing feature of the duo-servo brake is that the ends of the shoes are connected by a link via which, when the brake is actuated, frictional drag on one shoe, termed the primary shoe, is applied on the other shoe, termed the secondary shoe. This arrangement permits generally equal braking effort for either direction of rotation of the wheel, and also permits production of a high torque output compared to other types of drum brakes. For this reason duo-servo brakes are commonly employed when strong braking power is required for low brake actuation effort.
When, as is common for parking brakes, brake actuation is effected by mechanical means, rather than by hydraulic means for example, brake shoes are contacted or engaged by rigid elements, which are in connection to one another and which as an assembly may be actuated by a cable attached to one of the elements and leading to a parking brake handle or pedal actuable by the driver of a vehicle. For a parking brake which makes use of rear-wheel brake assemblies, the hand brake handle or pedal is provided in the driving compartment of the vehicle and connects to a front cable which leads rearwards and connects, via an intermediate linkage for example, to rear cables connected to the brake shoe actuation assemblies associated with opposite rear wheels. Alternatively, cables may connect directly to the brake handle or pedal. There are two basic dispositions of a rear cable or the end portion of a cable connecting to a brake shoe actuation assembly, one disposition being that each rear cable or end portion of a cable lies generally parallel to the plane of rotation of a drum and wheel unit, and the other that a rear cable or cable end portion lies generally parallel to the rear axle and at a right-angle to the plane of wheel rotation. The former type of disposition is illustrated in FIG. 1 in which a drum brake assembly is shown in the plane of rotation of a wheel unit on which the brake assembly is mounted, rotation being clockwise as seen in the drawing for forward drive of a vehicle of which the wheel unit is a part. The drum brake assembly comprises a pair of shoes A and B which are provided in a drum D. The tips of the shoes A and B are connected by pull-off springs to an anchor pin and the ends thereof are pivoted on and connected by a link, for example an adjustable floating link, and also by a shoe-to-shoe spring. The shoes A and B normally are held out of contact with the drum due to the action of the pull-off springs, but may be moved outwards counter to the force of the pull-off springs and brought into contact with the drum by the action of a generally vertical lever A1 having an upper end connecting to an upper portion of shoe A and a generally horizontal cross-strut B1 having opposite ends connecting to upper end portions of lever A1 and of shoe B. A point P at the lower end of lever A1 is connected to an actuation cable C which extends therefrom in a line generally parallel to the plane of rotation of the drum and wheel unit and leads to a brake lever or pedal and which upon actuation of the brake lever or pedal is pulled forward in the direction indicated by the arrow in the drawing and pivots lever A1 counterclockwise, whereby cross-strut B1 is moved upwards and so forces the shoes A and B away from anchor pin and into contact with the drum to stop rotation of the wheel unit. This arrangement of the brake assembly and disposition of the actuation cable C are very effective in terms of efficiency of transmission of the force of the pull exerted on the cable C, but present problems in terms of installation in a practical vehicle, since while it is necessary that the cable C be generally parallel to the wheel unit, the cable C must not be contacted by or interfere with the various elements of the vehicle rear suspension assembly, and there are thus practical restrictions on mounting and use of such a brake means. Another disadvantage of the arrangement shown in FIG. 1 is that, even supposing that there are no restrictions on mounting of the actuation cable C and that the cable C can be disposed exactly in a required alignment, there is a difference in tautness of the cable C depending on whether the vehicle is travelling forwards or in reverse. This is because when the drum is rotating clockwise as seen in the drawing, i.e., when the vehicle is travelling forwards, as is well known, shoe A tends to be pushed onto the anchor pin while shoe B moves to a position wherein the tip thereof is removed from the anchor pin by a clearance of .theta..degree. measured with reference to the circle defined by the drum D. For optimum braking action during forward motion of the vehicle, the mounting positions of the lever A1 and cross-strut B relative to one another and to the shoes A and B are suitably made such that when the shoes A and B are in the abovenoted positions relative to the anchor pin the lever A1 is generally vertical and the cross-strut B1 is generally horizontal, as indicated by the solid line portions of FIG. 1. When however the vehicle is moved in reverse and the drum D is rotated counterclockwise as seen in the drawing, shoe B is moved counterclockwise to close the clearance .theta..degree. and is brought into contact with the anchor pin. When this happens, since the mounting points of the lever A1 and cross-strut B1 are moved out of their forward drive disposition to a new disposition, as indicated by the dotted line portion of FIG. 1, the lever is pivoted through an angle .theta..degree. and the cable connection point P thereof is thereby moved a distance .DELTA.s forwards, i.e., towards the brake actuation handle or pedal. In other words, during reverse travel of the vehicle, although the actual length of cable C extending between the brake assembly and the brake actuation lever remains unchanged, the distance over which the cable C is required to stretch is shortened by a distance .DELTA.s, and the cable C is therefore less taut, and the same amount of movement of the brake actuation handle or pedal results in a different braking action depending on whether the vehicle is travelling forwards or in reverse.
To avoid such problems of mounting of and alteration of the tension length of a brake actuation cable it has been known to provide a drum brake actuation means such as shown in FIG. 2. This brake actuation means is disposed horizontally in the upper portion of the interior of a brake drum 8 and comprises a first lever 1 and a second lever 2, both of which have the general shape of a reversed L, as seen from above, each lever 1 and 2 having an inner leg which extends into the drum 8 interior and is generally parallel to the rear axle of a vehicle, the first lever 1 having an outer leg which is generally parallel to the plane of rotation of the drum 8, and the second lever 2 having an outer leg which comprises a base portion which is parallel to the drum rotation plane, and connects to the inner leg of the second lever 2, and an extension portion which is inclined away from the drum rotation plane and extends far enough to underlap the junction of the inner and outer legs of the first lever. The levers 1 and 2 are connected by a connector pin 9 which passes through the outer end of the extension portion of the second lever 2 and the junction of the inner and outer legs of the first lever 1, the levers 1 and 2 being fixedly attached to the pin 9, and the pin 9 being unattached to any other brake elements.
The inner leg of the first lever 1 passes through an opening 4a formed in the backplate 4 of the brake assembly and defines a tip end 1a which extends into a hole 6a which is formed in a shoe 6 near an anchor pin 5 to which the shoe 6 is connected in the manner described above, the shoe 6 being associated with another shoe 7 provided on the opposite side of the anchor pin 5. To one side of the inner leg of the first lever 1 there is affixed a large-diameter support pin 1b which contacts and may slide on an inner surface of the backplate 4. The inner leg of the second lever 2 similarly extends through an opening 4b formed in the backplate 4, defines a tip end portion 2a extending into an opening 7a formed in the shoe 7 near to the anchor pin 5, and has affixed thereto a pin 2c which contacts and may slide on an inner surface of the backplate 4. The inner side of the base portion of the second lever 2 is in sliding contact with an outer wall of the backplate 4 and constitutes a sliding support surface 2b for the second lever 2. Thus the first lever is slidingly supported by pin 1b and the second lever 2 by support surface 2b and pin 2c. Attached to the end of the outer leg of the first lever 1 there is an actuation cable 3 which leads away from the wheel unit at approximately a right-angle thereto, i.e., along a line generally parallel to the vehicle rear axle, and after being passed round a guide, not shown, which is located at a convenient point in the rear portion of the vehicle, is led to and connected to a brake actuation handle or pedal which is provided in the driving compartment of the vehicle, and which when actuated by the driver of the vehicle causes the cable 3 to exert on the first lever 1 a pull P1 directed outwards with respect to the drum brake assembly, as indicated by the arrow in the drawing. This pull P1 causes the first lever 1 to pivot counterclockwise as seen in the drawing, whereby while the support pin 1b slides along the backplate 4 the tip 1a of the first lever 1 is moved in the direction of the arrow A and pushes the shoe 6 into contact with the drum 8. When thus turned, the first lever 1 turns the connector pin 9 to which both levers 1 and 2 are fixedly attached, and the pin 9 exerts a turning force on the second lever 2. However, because of contact of the sliding support surface 2b of the second lever 2 with the backplate 4 the second lever 2 may not be turned counterclockwise, but instead, while guided by the support surface 2b and pin 2c sliding on backplate 4 surfaces, is moved in the opposite direction to the first lever 1, as indicated by the arrow B, whereby the tip 7a thereof urges the shoe 7 into contact with the drum 8, thereby completing brake action.
This arrangement of a drum brake assembly has the advantages that mounting of a brake actuation cable is easier and that any displacement of brake shoes relative to the anchor pin during forward or reverse drive of the vehicle results in only a slight variation of tension in the brake actuation cable, whereby there is little change in the amount of brake handle or pedal effort required to obtain a particular braking force. However, the brake shown in FIG. 2 has a principal disadvantage in that since the levers 1 and 2 are supported by sliding support elements efficiency of transmission of pull P1 is very low, being of the order of 5%, which to some extent offsets the advantage of the duo-servo brake. Another disadvantage of such sliding support is of course that parts are subject to greater frictional wear and hence have reduced service life.